This invention relates to a method for preparing a supported hydroconversion catalyst, particularly with respect to improved hydrodenitrification (HDN) activity.
Considerable research has been undertaken to develop and commercialize new, cost effective, highly active hydroconversion catalysts for large volume first stage hydrocracking and catalytic cracking feed hydrotreater applications. The inclusion of significant amounts of 1000.degree. F..sup.+ residual components, including cracked stocks, in the hydrocarbon feeds to these processes has created severe activity and stability problems for the catalysts currently used commercially in these applications.
Commercial hydrodenitrification catalysts are well known. Currently the most active are Ni--Mo, Ni--W, and Co--mo supported on a gamma alumina carrier. The activity of these catalysts has been enhanced by the addition of phosphorus as a promoter. Haresnape, et al U.K. No. 701217 (1953), which is incorporated herein by reference, relates to an increase in hydrodesulfurization activity with the addition of phosphorus in the form of cobalt phosphomolybdate. Housam, et al U.K. No. 807583 (1959) relates to the promoting effect of phosphorus in improving hydrogenation activity for both Co--Mo/Al.sub.2 O.sub.3 and Ni--W/Al.sub.2 O.sub.3 catalysts.
While multiple impregnations can be used in preparing catalysts, a single step impregnation is desired. To obtain efficient impregnation and uniform distribution of metal on support, the metals must be maintained in solution during impregnation. Methods to maintain high concentrations of metals in solution are well known. Adams, U.S. Pat. No. 3,629,146, which is incorporated herein by reference, relates to a method for preparing a supported catalyst containing a molybdenum metal concentration in excess of 12%w (as metal), basis finished catalyst, in a single aqueous impregnation from a stabilized solution. To obtain efficient impregnation and uniform distribution of metal on a support, the metals must be maintained in solution during the impregnation. Precipitation of metals from the impregnating solution leads to non-uniform impregnation and loss of effectiveness of the deposited metal. Adams discloses that high concentrations of active molybdenum can be impregnated into a support by adding stabilizing amounts of hydrogen peroxide and phosphoric acid to the impregnating solution. We have found that hydrogen peroxide is not needed for all impregnating solutions since it is a solubilizing agent that permits increased molybdenum contents in phospho-molybdenum solutions.
Gardner et al, U.S. Pat. No. 2,946,739, which is incorporated herein by reference, discloses that a "conventional" method of preparing a Co--Mo/Al.sub.2 O.sub.3 hydrocracking catalyst includes impregnation with an aqueous solution of (NH.sub.4).sub.6 Mo.sub.7 O.sub.4.4H.sub.2 O in a 50/50 mixture of ethanolamine and water. The partially impregnated catalyst is then dried, calcined and impregnated in a second step with an aqueous solution of Co(NO.sub.3).sub.2 and Rh(NO.sub.3).sub.3. We have found that the combination of high concentrations of amine with phosphomolybdenum compounds causes unstable solutions, i.e., precipitation of metal compounds or mal-distribution of metals in the carrier.
During a study of various catalyst preparation methods we found that low concentrations of certain amine compounds which are soluble in the impregnation solution, such as amine alcohols, polyamines, and amine acids, in combination with at least one each of compounds of molybdenum or tungsten and of nickel or cobalt plus a phosphorus compound can be used to produce a stable impregnating solution. Typical amine alcohols are ethanolamine, propanolamines, butanolamines, diethanolamines, 2,2-diamino-1,3-propandiol, 2,2,3-triamino-1-propanol or triethanolamine. A typical polyamine is ethylenediamine. Typical amine acids are glycine and nitrilotriacetic acid. Hydrogen peroxide may also be used to aid in solution preparation, in some cases. Surprisingly we found that the HDN activity of a catalyst prepared with a solution containing both a phosphorus compound and an amine compound was more than 19% greater than a catalyst prepared with only a phosphorus compound. The amount of amine compound needed in this case was also substantially less than that taught in the prior art (see Gardner et al, U.S. Pat. No. 2,946,739). A catalyst prepared with only the amine compound and no phosphorus compound would be considerably less active for hydrodenitrification than the catalyst containing both phosphorus and amine.